"We know there's going to be some [oil] exposure, and we know there's an endangered species. If you put those two thing together, there is reason for concern," said Celine Godard-Codding, an environmental toxicologist at Texas Tech University.

A 2009 stock assessment report by the National Oceanic and Atmospheric Administration (NOAA) estimated that the potential biological removal, or PBR, level for the Gulf of Mexico sperm whale population is three.

That means the whales' long-term survival is at risk if, in addition to natural deaths, three sperm whales a year are killed or removed by human causes.

The loss of a handful of whales each year can impact a population of hundreds, because sperm whales—especially females—require a very long time to reach sexual maturity. Females then give birth to just three or four calves during their entire lifetimes.

"They're like humans. Most of the human population is not going to have six kids at once and do that every year," Godard-Codding said.

"As soon as we get to the level of three deaths caused by human interaction—and this would include the oil spill—that would jeopardize that particular sperm whale population."

Whales May Be Choked, Drowned, and Poisoned

Oil spills can affect sperm whales and other cetaceans, including dolphins, in a number of ways.

For starters, the marine mammals have to surface to breathe, and if they come up through an oil slick, they can suck the toxic substance into their lungs.

Previous studies have shown that at least some of the Gulf of Mexico sperm whales are known to hang around where the Deepwater Horizon oil rig was located before it exploded on April 20, triggering the spill.

"Between 2000 and 2005, about 300 [sperm] whales were seen on a consistent basis right in that area," Texas Tech's Godard-Codding said.

After the Exxon Valdez disaster, some populations of killer whales were reduced by as much as 40 percent, according to a 2008 study led by marine biologist Craig Matkin of the North Gulf Oceanic Society in Alaska.

Even now, that killer whale population has yet to recover and will likely go extinct in a few decades, Matkin said.

"We lost so many females out of that group that they couldn't catch up again. They still haven't caught up," he said.

If the current oil spill causes more than three Gulf sperm whale deaths this year, it could push that group into the "red zone," Matkin said.